Kha Shing is your typical Taiwanese yard. You will find all sorts of opinions about that. Anyway, get a good survey is ultimately the bottom line. Also, if you HAVE TO HAVE this boat, I would get with the owner and see what kind of cost is involved with putting remans in there. You could get the boat for a LOT less with existing engines and just repair the one. OR you could have your say into what kind of engines YOU would like to put in there and maybe settle on a price for that to happen. Those engines are likely 4BTs....the 4 cylinder version of the 6BTs. That boat, at the very least, likely has a semi-planing hull and quite likely has a modified V true planing hull. Would you like speed??? If so, a bit more power would go a long way. It seems 300hp would be right at the point of having too much power for a displacement speed boat and not enough to get you over the hump to planing speeds.
BTW, I think his asking price is high considering the market regardless of how new the engines are. A VERY similar boat that I have been watching for 25 grand less....
http://www.yachtworld.com/boats/1989.../United-States
Jeffersons have a tendency to be the better of the Taiwanese built boats. Honestly don't know how they do their QC, but they do have a consistently better product than the other "farmed out" Tawainese builders.
PS....I am a big fan of the sedans!!! I like the one level of living!!
Also, WELCOME ABOARD!!!